1. Field Of The Invention
This invention relates to a method of applying terminations to the opposed ends of ceramic bodies, and more particularly to a method of applying terminations to a multiplicity of ceramic capacitors in a single operation.
2. The Prior Art
Multilayer ceramic capacitors (MLC) typically comprise a plurality of layers of ceramic dielectric having electrode layers interposed between the ceramic layers. Typically, alternate electrode layers are exposed at opposite ends of the ceramic monolith. Typical examples of MLC's of the type described may be found in one or more of the following U.S. Pat. No. 3,004,197 Oct. 10, 1961 and U.S. Pat. No. 3,235,939 Feb. 29, 1966. After the capacitors are formed it is necessary to apply terminations to the exposed electrode portions so as to permit the capacitor to be readily connected to a printed circuit board or the like.
In the typical procedure of manufacturing an MLC, termination is effected by applying to the ends of the ceramic monolith having exposed electrodes, a paste which comprises low melt glass frit and conductive material, typically silver. The capacitor is thereafter heated to drive off solvents and/or binders to fuse the glass material and cause the silver component to make contact with the exposed areas of the metallic electrodes.
The thus terminated capacitors may thereafter have leads soldered to them.
The described method incorporates a number of drawbacks, including the relatively high electrical resistance of the terminating compound, the relative fragility of the compound, the possibility that the terminating material may not form an electrical connection to one or more of the electrode layers, the requirement of reheating the entire capacitor to effect melting of the frit components, the relatively high cost of silver necessarily included in the termination compound, and the difficulty of applying the termination paste accurately to the end portions of the capacitors which may in some instance be an eighth of an inch in dimension or less.
It is heretofore been proposed, in accordance with U.S. Pat. No. 3,992,761, to terminate capacitors by a plating operation. In accordance with the teaching of the last mentioned patent a multiplicity of capacitors are imbedded in a plastic block with the edge portions having exposed electrodes extending transversely of the block. Portions of the block are dissolved away, following which the block is immersed in a plating solution. When sufficient metal has been deposited over the end portions of the capacitors to effect electrical connections to the electrodes of the capacitors, the block is removed, the platic components dissolved away, and conductive leads are applied to the metalized end portions of the capacitors.
It has been proposed to effect termination by a vacuum deposition method know generally in the trade as sputtering. The sputtering process is advantageous in that through the process it is possible to deposit extremely thin layers of metallic material, with the assurance that all portions subjected to the metal deposition procedure will be intimately engaged by the deposited metal and thus all layers of the capacitor are contacted by the termination material.
A difficulty inherent in sputter application of termination materials, resides in the fact that deposited increments of metal will be received by all exposed portions of the capacitors. Thus, unless the side faces of the capacitor, i.e. the faces between the ends to which termination is to be effected, are not completely shielded from the sputtering operation, there is substantial likelihood for the formation of a shorting band or film of sputtered material extending between the end portions of the capacitor with resultant shunting or short-circuiting of the capacitor.
In order to render a sputtering procedure feasible as a means of terminating capacitors, it is imperative, for an economic operation, that hundreds or even thousands of capacitors be simultaneously treated. While conceptually, a sputtering operation could be simultaneously effected on a multiplicity of capacitors imbedded in a cast plastic block or the like as suggested in the above referenced U.S. Pat. No. 3,992,761, the difficulties in aligning the capacitors, casting of the block, removal of the surface portions of the block to expose the terminal ends of the capacitors etc. and dissolution of the block after sputter applications, would render such method impracticable. While it is likewise theoretically possible to effect sputter termination by fitting a multiplicity of capacitors into a specially formed metallic jig or die which so closely embraces the sides of the capacitors as to preclude the formation of a shunting band of sputtered material during metal deposition, the fabrication and loading of a die of such precise dimensions capable of handling large quantities of capacitors at a single run renders such method commercially useless.
Attempts have been made to effect a sputtering operation while shielding the portions of the capacitors which are to remain free of sputtered material by implanting the capacitors in an elastomeric block or slab having throughgoing apertures sized to intimately engage side portions of the capacitors while exposing their ends.
It has been discovered, however, that while such elastomeric material forms an adequate shield, such material tends to "out-gas" in the course of the sputtering operation which is necessarily carried out under vacuum conditions. The result of such "out-gasing" is the formation at the interface between the deposited sputtered material and the capacitors, of glassy increments or inclusions. The increments or inclusions result in insulating one or more of the layers of the capacitor from the sputtered material with the result that the capacitive value of the terminated device cannot be accurately predicted.